Patient reference tool

ABSTRACT

A patient reference device is provided for use during a medical procedure. The patient reference device includes a housing having a back side and a front side, at least three tracking markers attached to the front side of the housing, the housing extending around the at least three tracking markers and beyond a horizontal plane defined by tops of the at least three tracking markers, the housing terminating at a substantially continuous edge, and a sterile cover attached to the substantially continuous edge of the housing for covering the housing and the tracking markers.

TECHNICAL FIELD

The present disclosure is generally related to image guided medicalprocedures, and more specifically to a patient reference tool.

BACKGROUND

The present disclosure is generally related to image guided medicalprocedures using a surgical instrument, such as a fibre optic scope, anoptical coherence tomography (OCT) probe, a micro ultrasound transducer,an electronic sensor or stimulator, or an access port based surgery.

In the example of a port-based surgery, a surgeon or robotic surgicalsystem may perform a surgical procedure involving tumor resection inwhich the residual tumor remaining after is minimized, while alsominimizing the trauma to the intact white and grey matter of the brain.In such procedures, trauma may occur, for example, due to contact withthe access port, stress to the brain matter, unintentional impact withsurgical devices, and/or accidental resection of healthy tissue. A keyto minimizing trauma is ensuring that the spatial reference of thepatient as understood by the surgical system is as accurate as possible.

FIG. 1 illustrates the insertion of an access port into a human brain,for providing access to internal brain tissue during a medicalprocedure. In FIG. 1, access port 12 is inserted into a human brain 10,providing access to internal brain tissue. Access port 12 may includesuch instruments as catheters, surgical probes, or cylindrical portssuch as the NICO Brain Path. Surgical tools and instruments may then beinserted within the lumen of the access port in order to performsurgical, diagnostic or therapeutic procedures, such as resecting tumorsas necessary. The present disclosure applies equally well to catheters,DBS needles, a biopsy procedure, and also to biopsies and/or cathetersin other medical procedures performed on other parts of the body.

In the example of a port-based surgery, a straight or linear access port12 is typically guided down a sulci path of the brain. Surgicalinstruments would then be inserted down the access port 12.

Optical tracking systems, used in the medical procedure, track theposition of a part of the instrument that is within line-of-site of theoptical tracking camera. These optical tracking systems also require areference to the patient to know where the instrument is relative to thetarget (e.g., a tumour) of the medical procedure.

Conventional systems have shortcomings with respect to establishing thereference to the patient because conventional systems create problemswith either sterility or accuracy. Therefore, there is a need for animproved patient reference tool.

SUMMARY

One aspect of the present disclosure provides a patient reference devicefor use during a medical procedure. The patient reference devicecomprises a housing having a back side and a front side, at least threetracking markers attached to the front side of the housing, the housingextending around the at least three tracking markers and beyond ahorizontal plane defined by tops of the at least three tracking markers,the housing terminating at a substantially continuous edge, and asterile cover attached to the substantially continuous edge of thehousing for covering the housing and the tracking markers.

Another aspect of the present disclosure provides a medical navigationsystem comprising a patient reference device for use during a medicalprocedure, a sensor attached to the patient reference device, and acontroller at least electrically coupled to the sensor. The sensorprovides a signal to the controller indicating movement of the patientreference device. The patient reference device has a housing having aback side and a front side, at least three tracking markers attached tothe front side of the housing, the housing extending around the at leastthree tracking markers and beyond a horizontal plane defined by tops ofthe at least three tracking markers, the housing terminating at asubstantially continuous edge.

Another aspect of the present disclosure provides a patient referencedevice for use during a medical procedure. The patient reference devicecomprises a housing having a back side and a front side, at least onetracking marker attached to the front side of the housing, the housingextending around the at least one tracking marker and beyond a top ofthe at least one tracking marker, the housing terminating at asubstantially continuous edge, and a sterile cover attached to thesubstantially continuous edge of the housing for covering the housingand the tracking marker.

Another aspect of the present disclosure provides a kit comprising partsthat can be assembled to form the patient reference device. The kit mayinclude instructions for assembling the parts.

A further understanding of the functional and advantageous aspects ofthe disclosure can be realized by reference to the following detaileddescription and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the drawings, in which:

FIG. 1 illustrates the insertion of an access port into a human brain,for providing access to internal brain tissue during a medicalprocedure;

FIG. 2 shows an exemplary navigation system to support minimallyinvasive access port-based surgery;

FIG. 3 is a block diagram illustrating a control and processing systemthat may be used in the navigation system shown in FIG. 2;

FIG. 4A is a flow chart illustrating a method involved in a surgicalprocedure using the navigation system of FIG. 2;

FIG. 4B is a flow chart illustrating a method of registering a patientfor a surgical procedure as outlined in FIG. 4A;

FIG. 5 is a perspective drawing illustrating an exemplary arm forholding a patient reference device;

FIG. 6A is a perspective drawing illustrating an exemplary patientreference device with cover detached;

FIG. 6B is a perspective drawing illustrating an exemplary patientreference device with cover attached;

FIG. 7 is a front view of the exemplary patient reference device shownin FIG. 6B;

FIG. 8 is a side view of the exemplary patient reference device shown inFIG. 6B;

FIG. 9A is a perspective drawing illustrating an environmental contextof the exemplary patient reference device shown in FIG. 6;

FIG. 9B is a perspective drawing showing the exemplary patient referencedevice shown in FIG. 6 installed;

FIG. 9C is a perspective drawing showing the exemplary patient referencedevice shown in FIG. 6 installed;

FIG. 9D is a perspective drawing showing the exemplary patient referencedevice shown in FIG. 6 installed with sterile draping in place; and

FIG. 9E is a perspective drawing showing the exemplary patient referencedevice shown in FIG. 6 installed with sterile draping in place.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

As used herein, the terms, “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms,“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

As used herein, the term “exemplary” means “serving as an example,instance, or illustration,” and should not be construed as preferred oradvantageous over other configurations disclosed herein.

As used herein, the terms “about” and “approximately” are meant to covervariations that may exist in the upper and lower limits of the ranges ofvalues, such as variations in properties, parameters, and dimensions. Inone non-limiting example, the terms “about” and “approximately” meanplus or minus 10 percent or less.

Unless defined otherwise, all technical and scientific terms used hereinare intended to have the same meaning as commonly understood by one ofordinary skill in the art. Unless otherwise indicated, such as throughcontext, as used herein, the following terms are intended to have thefollowing meanings:

As used herein, the phrase “access port” refers to a cannula, conduit,sheath, port, tube, or other structure that is insertable into asubject, in order to provide access to internal tissue, organs, or otherbiological substances. In some embodiments, an access port may directlyexpose internal tissue, for example, via an opening or aperture at adistal end thereof, and/or via an opening or aperture at an intermediatelocation along a length thereof. In other embodiments, an access portmay provide indirect access, via one or more surfaces that aretransparent, or partially transparent, to one or more forms of energy orradiation, such as, but not limited to, electromagnetic waves andacoustic waves.

As used herein the phrase “intraoperative” refers to an action, process,method, event or step that occurs or is carried out during at least aportion of a medical procedure. Intraoperative, as defined herein, isnot limited to surgical procedures, and may refer to other types ofmedical procedures, such as diagnostic and therapeutic procedures.

Embodiments of the present disclosure provide imaging devices that areinsertable into a subject or patient for imaging internal tissues, andmethods of use thereof. Some embodiments of the present disclosurerelate to minimally invasive medical procedures that are performed viaan access port, whereby surgery, diagnostic imaging, therapy, or othermedical procedures (e.g. minimally invasive medical procedures) areperformed based on access to internal tissue through the access port.

Referring to FIG. 2, an exemplary navigation system environment 200 isshown, which may be used to support navigated image-guided surgery. Asshown in FIG. 2, surgeon 201 conducts a surgery on a patient 202 in anoperating room (OR) environment. A medical navigation system 205comprising an equipment tower, tracking system, displays and trackedinstruments assist the surgeon 201 during his procedure. An operator 203is also present to operate, control and provide assistance for themedical navigation system 205.

Referring to FIG. 3, a block diagram is shown illustrating a control andprocessing system 300 that may be used in the medical navigation system200 shown in FIG. 3 (e.g., as part of the equipment tower). As shown inFIG. 3, in one example, control and processing system 300 may includeone or more processors 302, a memory 304, a system bus 306, one or moreinput/output interfaces 308, a communications interface 310, and storagedevice 312. Control and processing system 300 may be interfaced withother external devices, such as tracking system 321, data storage 342,and external user input and output devices 344, which may include, forexample, one or more of a display, keyboard, mouse, sensors attached tomedical equipment, foot pedal, and microphone and speaker. Data storage342 may be any suitable data storage device, such as a local or remotecomputing device (e.g. a computer, hard drive, digital media device, orserver) having a database stored thereon. In the example shown in FIG.3, data storage device 342 includes identification data 350 foridentifying one or more medical instruments 360 and configuration data352 that associates customized configuration parameters with one or moremedical instruments 360. Data storage device 342 may also includepreoperative image data 354 and/or medical procedure planning data 356.Although data storage device 342 is shown as a single device in FIG. 3,it will be understood that in other embodiments, data storage device 342may be provided as multiple storage devices.

Medical instruments 360 are identifiable by control and processing unit300. Medical instruments 360 may be connected to and controlled bycontrol and processing unit 300, or medical instruments 360 may beoperated or otherwise employed independent of control and processingunit 300. Tracking system 321 may be employed to track one or more ofmedical instruments 360 and spatially register the one or more trackedmedical instruments to an intraoperative reference frame. For example,medical instruments 360 may include tracking spheres that may berecognizable by a tracking camera 307. In one example, the trackingcamera 307 may be an infrared (IR) tracking camera. In another example,as sheath placed over a medical instrument 360 may be connected to andcontrolled by control and processing unit 300.

Control and processing unit 300 may also interface with a number ofconfigurable devices, and may intraoperatively reconfigure one or moreof such devices based on configuration parameters obtained fromconfiguration data 352. Examples of devices 320, as shown in FIG. 3,include one or more external imaging devices 322, one or moreillumination devices 324, a robotic arm, one or more projection devices328, and one or more displays 205, 211.

Exemplary aspects of the disclosure can be implemented via processor(s)302 and/or memory 304. For example, the functionalities described hereincan be partially implemented via hardware logic in processor 302 andpartially using the instructions stored in memory 304, as one or moreprocessing modules or engines 370. Example processing modules include,but are not limited to, user interface engine 372, tracking module 374,motor controller 376, image processing engine 378, image registrationengine 380, procedure planning engine 382, navigation engine 384, andcontext analysis module 386. While the example processing modules areshown separately in FIG. 3, in one example the processing modules 370may be stored in the memory 304 and the processing modules may becollectively referred to as processing modules 370.

It is to be understood that the system is not intended to be limited tothe components shown in FIG. 3. One or more components of the controland processing system 300 may be provided as an external component ordevice. In one example, navigation module 384 may be provided as anexternal navigation system that is integrated with control andprocessing system 300.

Some embodiments may be implemented using processor 302 withoutadditional instructions stored in memory 304. Some embodiments may beimplemented using the instructions stored in memory 304 for execution byone or more general purpose microprocessors. Thus, the disclosure is notlimited to a specific configuration of hardware and/or software.

While some embodiments can be implemented in fully functioning computersand computer systems, various embodiments are capable of beingdistributed as a computing product in a variety of forms and are capableof being applied regardless of the particular type of machine orcomputer readable media used to actually effect the distribution.

At least some aspects disclosed can be embodied, at least in part, insoftware. That is, the techniques may be carried out in a computersystem or other data processing system in response to its processor,such as a microprocessor, executing sequences of instructions containedin a memory, such as ROM, volatile RAM, non-volatile memory, cache or aremote storage device.

A computer readable storage medium can be used to store software anddata which, when executed by a data processing system, causes the systemto perform various methods. The executable software and data may bestored in various places including for example ROM, volatile RAM,nonvolatile memory and/or cache. Portions of this software and/or datamay be stored in any one of these storage devices.

Examples of computer-readable storage media include, but are not limitedto, recordable and non-recordable type media such as volatile andnon-volatile memory devices, read only memory (ROM), random accessmemory (RAM), flash memory devices, floppy and other removable disks,magnetic disk storage media, optical storage media (e.g., compact discs(CDs), digital versatile disks (DVDs), etc.), among others. Theinstructions may be embodied in digital and analog communication linksfor electrical, optical, acoustical or other forms of propagatedsignals, such as carrier waves, infrared signals, digital signals, andthe like. The storage medium may be the internet cloud, or a computerreadable storage medium such as a disc.

At least some of the methods described herein are capable of beingdistributed in a computer program product comprising a computer readablemedium that bears computer usable instructions for execution by one ormore processors, to perform aspects of the methods described. The mediummay be provided in various forms such as, but not limited to, one ormore diskettes, compact disks, tapes, chips, USB keys, external harddrives, wire-line transmissions, satellite transmissions, internettransmissions or downloads, magnetic and electronic storage media,digital and analog signals, and the like. The computer useableinstructions may also be in various forms, including compiled andnon-compiled code.

According to one aspect of the present application, one purpose of thenavigation system 205, which may include control and processing unit300, is to provide tools to the neurosurgeon that will lead to the mostinformed, least damaging neurosurgical operations. In addition toremoval of brain tumours and intracranial hemorrhages (ICH), thenavigation system 205 can also be applied to a brain biopsy, afunctional/deep-brain stimulation, a catheter/shunt placement procedure,open craniotomies, endonasal/skull-based/ENT, spine procedures, andother parts of the body such as breast biopsies, liver biopsies, etc.While several examples have been provided, aspects of the presentdisclosure may be applied to any suitable medical procedure.

Referring to FIG. 4A, a flow chart is shown illustrating a method 400 ofperforming a port-based surgical procedure using a navigation system,such as the medical navigation system 200 described in relation to FIG.2. At a first block 402, the port-based surgical plan is imported. Adetailed description of the process to create and select a surgical planis outlined in the international publication WO/2014/139024 “PLANNING,NAVIGATION AND SIMULATION SYSTEMS AND METHODS FOR MINIMALLY INVASIVETHERAPY”, which claims priority to U.S. Provisional Patent ApplicationSer. Nos. 61/800,155 and 61/924,993, which are both hereby incorporatedby reference in their entirety.

Once the plan has been imported into the navigation system at the block402, the patient is affixed into position using a body holdingmechanism. The head position is also confirmed with the patient plan inthe navigation system (block 404), which in one example may beimplemented by the computer or controller forming part of the equipmenttower 201.

Next, registration of the patient is initiated (block 406). The phrase“registration” or “image registration” refers to the process oftransforming different sets of data into one coordinate system. Data mayincludes multiple photographs, data from different sensors, times,depths, or viewpoints. The process of “registration” is used in thepresent application for medical imaging in which images from differentimaging modalities are co-registered. Registration is used in order tobe able to compare or integrate the data obtained from these differentmodalities.

Those skilled in the relevant arts will appreciate that there arenumerous registration techniques available and one or more of thetechniques may be applied to the present example. Non-limiting examplesinclude intensity-based methods that compare intensity patterns inimages via correlation metrics, while feature-based methods findcorrespondence between image features such as points, lines, andcontours. Image registration methods may also be classified according tothe transformation models they use to relate the target image space tothe reference image space. Another classification can be made betweensingle-modality and multi-modality methods. Single-modality methodstypically register images in the same modality acquired by the samescanner or sensor type, for example, a series of magnetic resonance (MR)images may be co-registered, while multi-modality registration methodsare used to register images acquired by different scanner or sensortypes, for example in magnetic resonance imaging (MRI) and positronemission tomography (PET). In the present disclosure, multi-modalityregistration methods may be used in medical imaging of the head and/orbrain as images of a subject are frequently obtained from differentscanners. Examples include registration of brain computerized tomography(CT)/MRI images or PET/CT images for tumor localization, registration ofcontrast-enhanced CT images against non-contrast-enhanced CT images, andregistration of ultrasound and CT.

Referring now to FIG. 4B, a flow chart is shown illustrating a methodinvolved in registration block 406 as outlined in FIG. 4A, in greaterdetail. If the use of fiducial touch points (440) is contemplated, themethod involves first identifying fiducials on images (block 442), thentouching the touch points with a tracked instrument (block 444). Next,the navigation system computes the registration to reference markers(block 446).

Alternately, registration can also be completed by conducting a surfacescan procedure (block 450). The block 450 is presented to show analternative approach, but may not typically be used when using afiducial pointer. First, the face is scanned using a 3D scanner (block452). Next, the face surface is extracted from MR/CT data (block 454).Finally, surfaces are matched to determine registration data points(block 456).

Upon completion of either the fiducial touch points (440) or surfacescan (450) procedures, the data extracted is computed and used toconfirm registration at block 408, shown in FIG. 4A.

Referring back to FIG. 4A, once registration is confirmed (block 408),the patient is draped (block 410). Typically, draping involves coveringthe patient and surrounding areas with a sterile barrier to create andmaintain a sterile field during the surgical procedure. The purpose ofdraping is to eliminate the passage of microorganisms (e.g., bacteria)between non-sterile and sterile areas. At this point, conventionalNavigation systems require that the non-sterile patient reference isreplaced with a sterile patient reference of identical geometry locationand orientation. Numerous mechanical methods may be used to minimize thedisplacement of the new sterile patient reference relative to thenon-sterile one that was used for registration but it is inevitable thatsome error will exist. This error directly translates into registrationerror between the surgical field and pre-surgical images. In fact, thefurther away points of interest are from the patient reference, theworse the error will be.

Upon completion of draping (block 410), the patient engagement pointsare confirmed (block 412) and then the craniotomy is prepared andplanned (block 414).

Upon completion of the preparation and planning of the craniotomy (block414), the craniotomy is cut and a bone flap is temporarily removed fromthe skull to access the brain (block 416). Registration data is updatedwith the navigation system at this point (block 422).

Next, the engagement within craniotomy and the motion range areconfirmed (block 418). Next, the procedure advances to cutting the duraat the engagement points and identifying the sulcus (block 420).

Thereafter, the cannulation process is initiated (block 424).Cannulation involves inserting a port into the brain, typically along asulci path as identified at 420, along a trajectory plan. Cannulation istypically an iterative process that involves repeating the steps ofaligning the port on engagement and setting the planned trajectory(block 432) and then cannulating to the target depth (block 434) untilthe complete trajectory plan is executed (block 424).

Once cannulation is complete, the surgeon then performs resection (block426) to remove part of the brain and/or tumor of interest. The surgeonthen decannulates (block 428) by removing the port and any trackinginstruments from the brain. Finally, the surgeon closes the dura andcompletes the craniotomy (block 430). Some aspects of FIG. 4A arespecific to port-based surgery, such portions of blocks 428, 420, and434, but the appropriate portions of these blocks may be skipped orsuitably modified when performing non-port based surgery.

When performing a surgical procedure using a medical navigation system200, as outlined in connection with FIGS. 4A and 4B, the medicalnavigation system 200 must acquire and maintain a reference of thelocation of the tools in use as well as the patient in three dimensional(3D) space. In other words, during a navigated neurosurgery, there needsto be a tracked reference frame that is fixed relative to the patient'sskull. During the registration phase of a navigated neurosurgery (e.g.,the step 406 shown in FIGS. 4A and 4B), a transformation is calculatedthat maps the frame of reference of preoperative MRI or CT imagery tothe physical space of the surgery, specifically the patient's head. Thismay be accomplished by the navigation system 200 tracking locations offiducial markers fixed to the patient's head, relative to the staticpatient reference frame. The patient reference frame is typicallyrigidly attached to the head fixation device, such as a Mayfield clamp.Registration is typically performed before the sterile field has beenestablished (e.g., the step 410 shown in FIG. 4A).

Most conventional navigation systems require the patient reference beexchanged during the draping phase (e.g., step 410) and the originalpatient reference frame used for registration is replaced with a sterilepatient reference frame. This exchange can cause a loss of accuracy.

Other conventional systems may require the non-sterile reference frameto be draped with a sterile, transparent plastic surgical drape. Wheretracking spheres are used in conjunction with an infrared (IR) trackingcamera, visibility through this drape can cause optical distortion ofthe measured reference position and can cause loss of accuracy. Thisprocess is also operator and set-up dependent, being affected by how thesterile drape is positioned and how tightly it is formed around thereference frame.

Also, throughout a navigated surgery, the patient reference frame issometimes bumped by the surgeon or others involved into the procedure. Abump that is strong enough could cause a shift in the frame's locationand therefore create a misregistration.

In order to address the shortcomings of conventional systems outlinedabove, according to one aspect of the present description, a patientreference design is provided that incorporates a removable sterilecover. According to another aspect of the present description, a sensormay be attached to or embedded in the patient reference frame to providethe medical navigation system 200 with information that can be used todetermine when the patient reference frame is bumped with enough forcethat the frame's location may have to be re-registered.

The sterile drape may include a plastic lens that is placed over thepatient reference face containing the tracking markers. In one example,the sterile cover maybe a substantially rigid lens. In one example, themarkers could be active IR markers or passive reflective spheres. Thesterile cover may not cause significant distortion like a standard drapewould. The sterile cover may have a transparent plastic sock thatextends downward from the cover to cover the rest of the patientreference and patient reference mounting arm and extension. The patientreference may be designed to permit +/−45 degree line-of-sight betweenthe tracking camera 307 (e.g., a Northern Digital Polaris Spectra) andthe patient reference.

Force Sensors and/or accelerometers may be connected to the medicalnavigation system 200, either wired or wirelessly, and the medicalnavigation system 200 may display a warning and/or force re-registrationif too great of a force and/or acceleration is imparted on the patientreference.

Referring to FIG. 5, an exemplary arm 500 is shown for holding a patientreference device. Arm 500 may also be referred to a patient referencearm 500. Patient reference arm 500 may include a tightening screw 502for securing the patient reference arm 500 once patient reference arm500 has been suitably adjusted. Patient reference arm 500 may attach toa Mayfield head holder or other suitable head holding device usingmounting bolt 504. In another example, patient reference arm 500 mayattach directly to a patient bed so long as the spatial relationshipbetween the receiving device for mounting bolt 504 and the patient'shead is static and known by medical navigation system 200. The mountingbolt 504 may secure the patient reference arm 500 to a Mayfield clamp.Once the tightening screw 502 is tightened the arm 500 may not pivot;the user may clock the arm 500 to his desired position using thestarbust connection. Once the screw 502 is tightened, a rigid connectionbetween the Mayfield clamp and the arm 500 is provided. Arm 500 alsoincludes housing 506. While one example of an arm 500 for connecting apatient reference device with a head holding device has been shown, anysuitable arm or connecting mechanism may be used according to the designcriteria of a particular application.

Referring to FIGS. 6A and 6B, an exemplary patient reference device 600is shown according to one aspect of the present application. FIG. 6ashows a perspective drawing illustrating the patient reference device600 with a cover detached. FIG. 6b shows a perspective drawing of thepatient reference device 600 with the cover attached. FIG. 7 shows afront view of the patient reference device with the drape attached. FIG.8 shows a side view of the patient reference device 600 with the drapeattached. FIGS. 6-8 are now discussed concurrently.

The patient reference device 600 may be used during a medical procedure.The patient reference device 600 includes a housing 602 having a backside 604 and a front side 606. In one example, at least three trackingmarkers 608 are attached to the front side 606 of the housing 602. Inanother example, four or more tracking makers 608 may be used. Thetracking markers 608 each have a top 614, generally on the opposite sidefrom where the tracking markers 608 attach to the housing 602. While anexample of either three or four tracking markers 608 is provided, anynumber of tracking markers 608 may be used to meet the design criteriaof a particular application. In one example, only one or two trackingmarkers may be used. In one example, the tracking markers 608 may bepassive reflective tracking spheres or active infrared (IR) markers thatmay be visible to a tracking camera, such as the tracking camera 307 ofthe medical navigation system 200. In another example, the trackingmarkers 608 may be active light emitting diodes (LEDs) or a graphicalpattern printed on a three dimensional (3D) structure used by a visionsystem such as the tracking camera to acquire 6 degrees of freedom(DOF).

The housing 602 shown in FIG. 6 is generally disc shaped. However, anysuitable shaped housing or frame may be used to meet the design criteriaof a particular application. In some examples, the housing 602 may be asolid member, either square shaped or disc shaped and the frame mayfurther have superfluous material removed that is not important to thestructural integrity of the housing (e.g., the housing may be generallysquare shaped or disc shaped with holes formed therein In one example,the housing 602 may be constructed of a metal such as machined aluminum,blasted with aluminum oxide (e.g., 180 grit), and then hard anodized.Both the blasting and anodization processes may result in a matte finishof the housing 602 that may minimize reflection seen by the trackingcamera. Naked metallic surfaces or even plastic sometimes lead to pooraccuracy for camera based tracking systems due to the presence ofreflection, which can be further magnified with the use of a plasticdrape on the patient reference 600. The exemplary blasted and anodizedaluminum finish may improve tracking performance of the tracking camerawithout degrading accuracy. While one example of a suitable finish forthe housing 602 is provided, any suitable finish of low reflectivity maybe used to meet the design criteria of a particular application. In oneexample, the housing 602 may be made of any suitable type of plastic ormetal.

The housing 602 shown in FIGS. 6-8 extends along the back side 604 ofthe housing 602. The housing 602 further extends beyond a horizontalplane defined by the tops 614 of the tracking markers 608. The housingterminates at an edge 610. In one example, the edge 610 may besubstantially continuous, such as forming a shape such as a circle, asquare, an oval, or a rectangle in one plane. A sterile cover 612 may beattached to the substantially continuous edge 610 of the housing 602 forcovering the housing 602 and the tracking markers 608. In one example,the housing 602 may be generally domed shaped with a flattened back sideand the sterile cover 612 may be round. However, the housing 602 mayalso be pyramid shaped, cone shaped, dome shaped, dish shaped, or of anyother suitable shape to meet the design criteria of a particularapplication. The shape of the sterile cover 612 is then designed to mateappropriately with the shape of the housing 602.

The housing 602 of the patient reference device 600 may be attachable toa patient reference arm, such as the patient reference arm 500 shown inFIG. 5. The patient reference arm 500 may be attachable by way of themounting bolt 510 to a Mayfield head holder or any other head securingdevice, such that the patient reference device 600 is rigidly attachedin a static location relative to the head securing device.

In one example, the continuous edge 610 may have a seal located on thecontinuous edge 610 for forming a seal between the housing 602 and thesterile cover 612. In one example, the seal may be attached to thecontinuous edge 610 using any suitable adhesive. The sterile cover 612may further have a sterile drape attached thereto for covering thehousing 602 and a patient reference arm 500 attached to and holding thepatient reference device 600 in position.

In one example, a lens 616 of the sterile cover 612 may be made of asubstantially transparent plastic material that can be easily sterilizedand has optical properties that are known and controlled such thatinfrared light passing through the lens 616 of the sterile cover 612 andreflecting off of the tracking markers 608 and passing back through thelens 616 of sterile cover 612 does so without excessive diffraction suchthat it becomes problematic for the tracking camera (e.g., the trackingcamera 307) that is monitoring the tracking markers 608. In one example,the sterile cover 612 could be made of glass, quartz, or sapphire. Insome examples, the lens 616 of the sterile cover 612 may have additionaloptical properties, such as that of a band pass filter that allowsinfrared light to pass through but blocks any suitable portion of thefrequency spectrum on each side of the IR pass band. In another example,the lens 616 of the sterile cover 612 may have the optical properties ofa low pass or a high pass optical filter. Alternatively, the opticalproperties of the lens 616 of the sterile cover 612 may be optimized forpassing visible light or only visible light in the example where agraphical pattern is printed on a structure. While some examples havebeen given for possible optical filter characteristics, any suitableoptical filter may be applied to the lens 616 to meet the designcriteria of a particular application.

The patient reference device 600 may further have one or more sensors(not shown) attached thereto for providing a signal to a medicalnavigation system, such as the medical navigation system 205 shown inFIG. 2, which may include the control and processing unit 300 shown inFIG. 3. In one example, the sensor may include, an accelerometer, aforce sensor, a gyroscope, a magnetometer, a strain gauge, or any othersuitable sensor. The sensor may be either attached to the exterior ofthe housing 602 or embedded in or integrated into the housing 602. Inone example, the patient reference device 600 may have a triaxialaccelerometer attached thereto for sensing acceleration in any of the X,Y, and Z directions and providing the signal generated by theaccelerometer to the control and processing unit 300. For example, theaccelerometer mounted on the patient reference device 600 may be shownin FIG. 3 as one of the external I/O devices 344. The control andprocessing unit 300 may be programmed (e.g., one of the processingengines 370) to monitor signals from the accelerometer after the patientreference device 600 has been put into position and registered duringthe registrations phases 406/408 (FIG. 4A).

When the control and processing system 300 detects an acceleration fromthe accelerometer that indicates that the patient reference device 600or the patient reference arm 500 has been jolted, perhaps by one of themedical staff accidentally hitting or kicking the patient referencedevice 600 or the patient reference arm 500, and when the accelerationindicated by the accelerometer exceeds a threshold such that enoughforce was generated that could have thrown the patient reference deviceout of its proper fixed position, the control and processing system 300may respond accordingly. For example, the control and processing system300 may display a warning on the display 311 to the operator to checkthe position of the patient reference device 600. In another example,the control and processing system 300 may simply require the operator ofthe system to reregister the patient reference device 600 to ensure thatthe position of the patient reference device 600 relative to the headholding device is properly understood by the medical navigation system200.

Referring now to FIG. 9A, a perspective drawing is shown illustrating anenvironmental context of the exemplary patient reference device 600shown in FIG. 6. In FIG. 9A, the patient reference device 600 is shownattached to an arm that fixes the patient reference device 600 inposition at the head end of a medical bed for performing a medicalprocedure in an operating room type environment, as discussed above.Several aspects of the navigation system 205 discussed in connectionwith FIG. 2 are shown surrounding the medical bed.

Referring now to FIG. 9B, a perspective drawing is shown illustratingthe exemplary patient reference device 600 installed. FIG. 9C is aperspective drawing showing the exemplary patient reference device 600installed. FIG. 9D is a perspective drawing showing the exemplarypatient reference device 600 installed with sterile draping in place.FIG. 9E is a perspective drawing showing the exemplary patient referencedevice 600 installed with sterile draping in place. FIGS. 9B-E are nowdiscussed concurrently.

In FIGS. 9B-D, the patient reference device is shown attached to an armsuch as the arm 500. The mounting bolt 504 of the arm 500 attaches thearm 500 to a Mayfield clamp 902, or any other suitable head holdingdevice for restraining the head of a patient. Since the patientreference device 600 is therefore rigidly attached to the Mayfield clamp902, the patient reference device 600 is located in a fixed locationrelative to the patient's head and therefore relative to the surgicalsite of interest.

FIGS. 9D-E show the patient reference device 600 with a sterile drape904 attached, covering arm 500 and other components that are typicallynot sterilized prior to the medical procedure to be performed. Sterilecover 612 and lens 616 are also shown in position attached to thecontinuous edge 610 of the patient reference device 600.

In one example, the patient reference device 600 may be sold as a kitfor assembly by an end user, such as a hospital or medical clinic. Thekit may include the housing 602, the cover 612, the arm 500 havingmounting bolt 504 or other suitable connecting mechanism for attachingto a Mayfield clamp, the tightening screw and/or knob 502, the trackingmarkers 608, and/or the sterile drape 904.

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover all modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

We claim:
 1. A patient reference device for use during a medicalprocedure, the patient reference device comprising: a housing having aback side and a front side; at least three tracking markers attached tothe front side of the housing, wherein the at least three trackingmarkers are comprised of passive reflective tracking spheres detectableby an infrared tracking camera; the housing extending around the atleast three tracking markers and beyond a horizontal plane defined bytops of the at least three tracking markers, the housing terminating ata substantially continuous edge; and a sterile cover attached to thesubstantially continuous edge of the housing for covering the housingand the tracking markers.
 2. The patient reference device according toclaim 1, wherein the housing is attachable to a patient reference armthat is attachable to a Mayfield head clamp.
 3. The patient referencedevice according to claim 1, wherein there are at least four trackingmarkers attached to the front side of the housing.
 4. The patientreference device according to claim 1, wherein the housing is domeshaped with a flattened back side.
 5. The patient reference deviceaccording to claim 1, further comprising a sensor attached thereto forproviding a signal to a medical navigation system.
 6. The patientreference device-according to claim 5, wherein the sensor includes atleast one of an accelerometer, a strain gauge, and a force sensor. 7.The patient reference device according to claim 1, wherein the sterilecover includes a lens made of a substantially rigid and substantiallytransparent plastic.
 8. The patient reference device according to claim7, wherein the lens has the properties of an optical band pass filter.9. The patient reference device according to claim 1, wherein thehousing is substantially dome shaped with a flattened back side and thesterile cover is substantially round.
 10. The patient reference deviceaccording to claim 1, further comprising: a seal located on thesubstantially continuous edge for forming a seal between the housing andthe sterile cover.
 11. The patient reference device according to claim10, wherein the sterile cover further includes a sterile drape attachedto the seal for covering the housing and wherein the sterile drape isconfigured to be attached to a patient reference arm.
 12. A kitcomprising parts that can be assembled to form the patient referencedevice according to claim 1, the kit including instructions forassembling the parts.
 13. A medical navigation system, comprising: atracking camera; a patient reference device for use during a medicalprocedure, the patient reference device having: a housing having a backside and a front side; and at least three tracking markers attached tothe front side of the housing, wherein the at least three trackingmarkers are configured for interacting with the tracking camera; thehousing extending around the at least three tracking markers and beyonda horizontal plane defined by tops of the at least three trackingmarkers, the housing terminating at a substantially continuous edge;wherein the patient reference device further includes a sterile coverattached to the substantially continuous edge of the housing forcovering the housing and the tracking markers; a sensor attached to thepatient reference device; and a controller at least electrically coupledto the sensor, the sensor providing a signal to the controllerindicating movement of the patient reference device relative to apatient.
 14. The medical navigation system according to claim 13,wherein the sensor includes at least one of an accelerometer and a forcesensor.
 15. A patient reference device for use during a medicalprocedure, the patient reference device comprising: a housing having aback side and a front side; at least one tracking marker attached to thefront side of the housing wherein the at least one tracking marker iscomprised of passive reflective tracking spheres detectable by aninfrared tracking camera; the housing extending around the at least onetracking marker and beyond a top of the at least one tracking marker,the housing terminating at a substantially continuous edge; and asterile cover attached to the substantially continuous edge of thehousing for covering the housing and the tracking marker.
 16. Thepatient reference device according to claim 15, wherein the sterilecover includes a lens made of a substantially rigid and substantiallytransparent plastic.
 17. The patient reference device according to claim15, wherein the housing is dome shaped with a flattened back and thesterile cover is substantially round.